1. Field of the invention
This invention relates in general to subsea well drilling, and in particular to a means for preventing an inner wellhead housing secured to the lower end of a riser suspended from a drill ship from rotating within a conductor or outer wellhead housing.
2. Description of the Prior Art
Many subsea wells are drilled by first drilling a large diameter hole, then installing a string of conductor pipe, which has an outer wellhead housing secured to the upper end. Then, the operator drills the well to a greater depth and installs a first string of casing. An inner wellhead housing secures to the upper end of the string of casing and lands within the outer wellhead housing. The operator will then drill the well to a further depth. A string of riser will extend from the inner wellhead housing to the drilling vessel.
If a drill ship is being used as a drilling vessel, the drill ship will swing into the wind. This swinging movement can cause rotational forces on the riser. Normally, the rotation is resisted by frictional engagement of the landing shoulders of the inner wellhead housing and the outer wellhead housing. If the rotational force is high enough to cause the inner wellhead housing to begin to rotate within the outer wellhead housing, one of the casing joints below the inner wellhead housing could start to unscrew, causing a serious problem.